A marine radar device transmits electromagnetic waves from a radar antenna, converts received data in a polar coordinate system that has been obtained from the electromagnetic waves reflected by a target into a Cartesian coordinate system, stores the data in a video memory, and then displays it on a display unit by a raster scanning method. Here, the received data may not always contain only components reflected by a desired target, but it may also contain unnecessary components resulting from sea clutter and the like (hereinafter, this is referred to as “clutter”). For this reason, conventional radar devices have a scan correlation processing function of repeatedly performing a correlation between the current received data obtained during one rotation of the radar antenna, and previous scan correlated data. By performing this process, it is possible to eliminate momentary data such as clutter, thus displaying only the received data from a desired target.
The structure of a conventional radar device performing such a scan correlation process will be described with reference to FIG. 9.
FIG. 9 is a block diagram showing the main part of a conventional radar device provided with scan correlation processing. While rotating through the horizontal plane at a predetermined rotation period, a radar antenna 101 sends out pulses of radio waves and receives, in a polar coordinate system, radio waves reflected by a target, at a predetermined send/receive period. It then outputs the received signal to a receiver 102, and outputs sweep angle data to a rendering address generator 105. The receiver 102 detects and amplifies the received signal from the radar antenna 101, and outputs it to an A/D converter 103. The A/D converter 103 converts this analog received signal into a digital signal (received data) constituted by a plurality of bits. A sweep memory 104 stores in real time the digitized received data for one sweep, and outputs this data for one sweep to a correlation data generator 107 before the received data obtained by the next send process is written again.
As shown in FIG. 2, the rendering address generator 105 generates addresses specifying pixels in the video memory, arranged in a corresponding Cartesian coordinate system, from the antenna angle θ (taking a predetermined direction, for example, the heading direction of a ship, as a reference) and the read-out position r of the sweep memory 104, taking the sweep rotation center as the start address and going around from the center. Specifically, the rendering address generator 105 is constructed by hardware that performs operations in accordance with the following equations:X=Xs+r·sin θY=Ys+r·cos θwhere X, Y: address specifying a pixel in the video memory
Xs, Ys: center address of sweep
r: distance from center
θ: angle of sweep (antenna)
A FIRST/LAST detector 106 detects the timing at which, in one sweep rotation (of the radar antenna 1), this sweep has first accessed or last accessed each pixel in the rectangular Cartesian coordinate system set in the rendering address generator 105 on a correlation processing video memory 108, and feeds one of those timings to a correlation data generator 107. Since it is necessary to limit the update of each pixel in the scan correlation processing video memory to once per sweep rotation, a FIRST signal or a LAST signal is used as this timing.
Based on the FIRST signal (or LAST signal) that is input from the FIRST/LAST detector 106, the correlation data generator 107 performs a scan correlation process using received data input from the sweep memory 104 and image data stored in the later-described correlation processing video memory 108 one sweep rotation before, and lets the correlation processing video memory 108 store the data again.
Here, for example, when the received data input from the sweep memory 104 is taken as N(t) and the correlated image data at the pixel position corresponding to the received data that has been obtained up to the previous rotation and that has been input from the correlation processing video memory 108 is taken as W(t−1), the scan correlation process calculates the correlated image data W(t) obtained in the current sweep using the following equation:W(t)=α·N(t)+β·W(t−1)
where α and β are any suitable numbers. The details of the scan correlation process can be changed by varying the values of α and β.
The correlation processing video memory 108 has sufficient capacity to store the received data (correlated image data) for one sweep rotation (of the radar antenna 1), and feeds back the correlated image data obtained one rotation before to the correlation data generator 107, for the purpose of the scan correlation process. Further, when a display unit 109 performs raster scanning under the control of a display controller (not shown), the correlation processing video memory 108 outputs the correlated image data in synchronization with this raster scanning. Here, by varying the brightness, the display color or the like in accordance with the data value of each pixel of the correlated image data, the operator can recognize the position, movement and the like of a target using this scan correlated image (e.g., see JP H11-352212A).
However, for the conventional radar device performing scan correlation processing, adjustment of GAIN, STC, FTC and the like during scan correlation requires a time corresponding to several rotations (depending on the values of α and β) of the above-described radar antenna until the result of adjustment is reflected by the images. During this time, it is difficult for the operator to determine an optimum amount of the adjustment, thus increasing the time required for the adjustment and making the adjustment difficult.
In addition, switching from a display with a raw received signal, which has not been subjected to the scan correlation process, to a display with scan correlated image data also requires a time corresponding to several rotations of the antenna, and therefore deficiencies as described above arise. Furthermore, when the details of the selected scan correlation process are not appropriate, there may be cases where images that should be suppressed are highlighted, and, conversely, images that should be highlighted are suppressed. Determining such a state and changing the process details to be optimum also requires a predetermined time as described above.
Moreover, the details of the scan correlation process are different for other ships moving at high speed and for stationary targets such as the seashore, so that the image of other ships moving at high speed may be suppressed when the details of the scan correlation process are set so as to highlight stationary targets. That is, moving targets are suppressed when stationary targets are highlighted, and conversely, stationary targets are suppressed when moving targets are highlighted.
It is an object of this invention to provide a radar or similar device with which a desired image can be promptly observed by making an image resulting from the currently received data that has not been subjected to scan correlation to gradually become similar to an image that has been subjected to scan correlation, or by making an image that has been subjected to a given scan correlation process to gradually become similar to an image that has been subjected to a different scan correlation process.
It is also an object of this invention to provide a radar or similar device with which a desired image can be promptly obserbed, for example, by outputting the currently received data and the scan correlated image data in parallel with each other, or by outputting two sets of image data that have been subjected to scan correlation processes with different details, in parallel with each other.